Due to a requirement of a Time Division Duplexing (TDD) service on a time technology, synchronous-Ethernet-based implementation of 1588 protocol gradually becomes a mainstream time synchronization technology in the communication industry.
However, a 1588 time synchronization technology theoretically depends on bidirectional symmetry of a middle path transmission delay between synchronization nodes, but bidirectional asymmetry exists in an existing network, that is, there exists a phenomenon of bidirectional length asymmetry caused by multiple reasons such as an optical cable fibre core error, an optical cable construction connection error, an optical cable failure connection error and a tail fibre error, thereby causing the transmission delay bidirectional asymmetry. Existing network data shows that it is highly probable that a time error caused by optical fibre asymmetry is more than 100 ms, therefore the asymmetry problem cannot be ignored during the construction of a time network.
At present, methods for an operating company to solve the asymmetry problem during time network deployment mainly include a 1588 testing instrument-based method, an optical fibre changeover method, a single-fibre bidirectional transmission method, an automatic ring network testing method and the like, but the methods have respective defects. The 1588 testing instrument-based method requires point-by-point compensation during network construction, and it is difficult to meet a network construction requirement of nodes in a large range; the method of realizing optical fibre changeover through an optical switch is difficult to compatible with existing device, and service damage may be caused during changeover; the single-fibre bidirectional transmission method has multiple problems about user habits and convenience in use; and the automatic ring network testing method is limited in adaptability, and can only make an error judgment about a constructed time network. The abovementioned methods for the existing network have respective defects, causing compensation for asymmetry caused by the 1588 time synchronization technology to become a problem urgent to be solved by the industry, which seriously influences the network deployment of time synchronization for a wireless TDD service.
The physical calculation of a difference between double optical fibres by measuring the distance between the optical fibres becomes a realistic solution direction. One solution is to constitute a distance measurement system by Rayleigh scattering and backscattering formed by Fresnel reflection during the transmission of optical pulses in the optical fibres through an Optical Time Domain Reflectometry (OTDR) technology; however, the OTDR technology involves long-distance measurement, and is high in measurement time and execution complexity, thereby causing slow response and further causing poor accuracy and reliability, and moreover, it is difficult to integrate the OTDR technology into an optical module.